1. Field of the Invention
The present invention relates to a composite ball bearing which and more particularly to a composite bearing structure is a combination of a single row ball bearing and a cylindrical bearing.
2. Description of the Prior Art
Single row ball bearings are simple in construction and low cost, and are therefore widely used for the bearings in the rotational support mechanism of the rotational axis in compact motors, etc. FIG. 17 shows a sectional view through a single row ball bearing. In the single row ball bearing shown in FIG. 17, when a single bearing 100 is used, the outer ring 101 freely inclines through an angle of .theta. on top of ball 102. As a result, when a rotating element (not shown) such as the hub of a spindle motor is mounted on outer ring 101, this rotating element will vibrate in both the axial and radial directions against the inner ring 103, resulting in an unstable rotation.
Similarly, if outer ring 101 is fixed and when a rotating body (not shown) such as the rotational shaft of a spindle motor is mounted on inner ring 103, this rotating body will vibrate in both the axial and radial directions against the outer ring 101, also resulting in an unstable rotation.
In order to prevent this vibratory rotation of the rotating element, two single row ball bearings 100 and 110 are fixed together in a parallel assembly as shown in FIG. 18, or, to further reduce the rotational vibration, a spacer 120 is mounted between the two single row ball bearings 100 and 110, increasing the space between ball bearing 100 and ball bearing 110, preventing rotational vibration and achieving stable high precision rotation as shown in FIG. 19.
However, in response to the trend for smaller size in many kinds of device, when a rotating element of more compact dimensions is to be employed, it is desired to reduce the dimension L in FIGS. 18 and 19 as much as possible. Accordingly, to make the dimensions as compact as possible, the arrangement is often used as shown in FIG. 18 where single row ball bearings 100 and 110 are mounted side by side, instead of the arrangement where a spacer 120 is used between single row ball bearings 100 and 110 as shown in FIG C. If it is desired to reduce the minimum dimensions even more than this, the only solution is to specially manufacture thin multiple row ball bearings. Even if such special thin multiple row ball bearings are used, the dimensions are limited by the size of the balls in the 2 side-by-side parallel rows.
A concrete example of this trend toward reducing the size of devices is the remarkable recent reduction in the size and thickness of floppy disk drives and hard disk drives for magnetic storage equipment. Demand is especially strong for more compact, thinner spindle motors for use in such drives. To respond to this demand for more compact, thinner spindle motors, a major point is to find some method for making the dimensions of the bearing section more compact (thinner). At present, the only way to meet these requirements is to use a side-by-side double row of ball bearings, or specially produced multiple row ball bearings as mentioned above, and these methods have their limitations. For the bearing on one side of a spindle motor, it is desirable to use a single ball bearing in order to make the motor thinner. However, this method will result in rotational vibration as described above and is therefore not practical. Also, the use of 2 single row ball bearings for this purpose would cause an undesirable cost increase.
The present invention has been accomplished in the light of the above-mentioned problems and it is an object of the invention to provide a bearing structure in which a single ball bearing is used in such a way as to prevent rotational vibration is the same way as if two ball bearings were used, and in addition to provide a low cost, thin type ball bearing structure.